Microfilm retrieval systems are used to display and print documents that have previously been recorded on microfilm. Although manufacturers recommend conditions for exposure and chemical processing of the film images, wide variations in these can occur in practice. This leads to variations in the contrast and optical density or opacity among documents recorded on microfilm.
Recently, electronic systems have been developed that include a digital scanner, image processing hardware and software, and electronic printer or CRT display. These systems typically scan the film image and encode the light and dark image information in, for example, two, four, sixteen, or up to 256 levels. Since the designer and user of the electronic retrieval equipment have no control over the contrast and density of the film to be scanned, some way must be used to ensure that the important image information is properly captured, stored and processed so that the document information is rendered correctly on a display or print.
Enhancement of an image based on the shape of its histogram can be accomplished by a histogram equalization method. The cumulative distribution function can be calculated and used to transform the original image. Image statistics based on the histogram of the image values have also been used to compute a tone reproduction function, as described in U.S. Pat. No. 4,642,683. The '683 patent discloses a method to improve the appearance of natural scene images that exhibit a bimodal tone value distribution due to back-lit, or close-up flash film exposure. A histogram of sample tone values is generated and statistical tests for the presence of a bimodal distribution is performed.
In U.S. Pat. No. 4,636,845, a method for processing photographic images that have been over or under exposed is described. This method uses a normalized histogram and performs a test of skewness. If the computed skewness exceeds a predetermined amount, the image contrast, or tone values, are adjusted by an amount equal to the value of a standard variate computed for the sample at the mode of the tone values of the sample. This is done using a tone reproduction function that is again scene-specific.
These existing techniques yield good results for natural scenes where the distribution functions of the important information can be assumed to be a standard normal, or the sum of standard normal distributions. For document images, however, these transformations which are unique for each image yield inconsistent results. This is particularly true if the digital image information has been corrupted due to an intermediate step involving, for example, optical lenses or microfilm granularity noise.
It is seen then that there exists a need for an image scanner system which can effectively transform document images from microfilm.